ARM: dts: bcm2837-rpi-3-b-plus: Clarify label for STATUS_LED

The RPI 3 B+ provides control to both LEDs (PWR and ACT). So append
the first letter of the LED color (like in the schematics) in order
to clarify this.

Signed-off-by: Stefan Wahren <stefan.wahren@i2se.com>
Tested-by: Peter Robinson <pbrobinson@gmail.com>
Reviewed-by: Eric Anholt <eric@anholt.net>